amsthmitalicmath-modetheorems
Consider the following theorem:
\begin{theorem}
If $x=0$, then $xy$ is equal to 0.
\end{theorem}
The first "0" will be typeset in upright typeface and the second one in italic. Including amsthm has no effect. Is this a bug? I personally think it look very weird.
I think I know how to fix this, and this question is more about the why it is not fixed by default. However, if you have a clever trick for avoiding this, feel free to share it.
The documentation on this is opaque, but includes an example use that starts
\theoremstyle{<some style>}
\newmdtheoremenv[linecolor=blue]{lemma}{Lemma}[section]
The crucial thing here is the \theoremstyle command, which is not mdframed based, but refers to the amsthm styling commands. So you need to load amsthm, and that has to be done before loading mdframed.
\documentclass[a4paper,10pt]{book}
\usepackage[brazilian]{babel}
\usepackage{amsthm}
\usepackage{mdframed}
\usepackage{amsmath}
\usepackage{xcolor}
\mdfdefinestyle{definitionSty}{backgroundcolor=blue!10, linewidth=0pt, innerleftmargin=3ex, innerrightmargin=3ex, innertopmargin=3ex, innermargin =+1cm, outermargin =+1cm}
\newcounter{definitionCounter}[chapter]
\numberwithin{definitionCounter}{chapter}
\theoremstyle{definition}
\newmdtheoremenv[style=definitionSty]{definition}[definitionCounter]{Defini\c{c}\~{a}o}
\begin{document}
\begin{definition}Medição, grandeza e medida
\begin{itemize}
\item medição: \textit{processo} pelo qual se mede algo
\item grandeza: \textit{propriedade} quantificada por medição
\item medida: \textit{resultado} do processo de medição
\end{itemize}
\end{definition}
\begin{definition}[Medição, grandeza e medida]\leavevmode
\begin{itemize}
\item medição: \textit{processo} pelo qual se mede algo
\item grandeza: \textit{propriedade} quantificada por medição
\item medida: \textit{resultado} do processo de medição
\end{itemize}
\end{definition}
\end{document}
Note that I am not quite sure how I should be interpreting your argument in the definition environment. Either print it straight as in the first version or put it in square brackets, then there is then an extra \leavevmode tweak needed because of the itemize.
Instead of loading amsthm you can load ntheorem, either with the amsthm option and still use \theoremstyle, or using its own styling commands. You can also use the thmtools interfaces to either of these packages.
Here's a solution with thmtools, which eases defining new theorem styles:
\documentclass{book}
\usepackage[utf8]{inputenc} \usepackage{fourier, erewhon, cabin}
\usepackage[table, x11names]{xcolor} \usepackage{mathtools}
\usepackage{amsthm, thmtools}
\usepackage{microtype} \SetTracking[no ligatures = {f}]{encoding = *}{50}
\declaretheoremstyle[%
headfont=\color{SteelBlue3}\sffamily\bfseries\lsstyle,
within=chapter, headpunct={\medskip}, postheadspace=\newline, notefont=\color{black}\mdseries, notebraces = {\quad}{},spaceabove = 8pt,spacebelow = 8pt]%
{sample}
\declaretheorem[name=Sample Problem, style=sample]{mypb}
\begin{document}
\setcounter{chapter}{3}
\begin{mypb}[Adding vectors in a drawing, orienteering]
Show the following assertion is true: %
\begin{equation*}
a = a
\end{equation*}
\end{mypb}
\end{document}
Best Answer
The reason it is in italic is because the theorem style is to typeset the text in italic. Just write
$0$to have the 0 be treated as math and thus be upright.